Temperature is one of the most commonly measured parameters in electronic systems. There are many reasons for measuring temperature of an electronic device. One reason is for providing accurate measurements of other parameters that are influenced by the temperature of the sensor itself. Another reason is for protection of power electronics from damage caused by over-temperature conditions, which can degrade the performance and reliability of the power electronics. For example, a temperature sensor may be integrated with a power electronics module to shut down the power to the module if temperature exceeds a critical value. Often, this is accomplished using a comparator and a temperature-sensing device. Typical temperature sensing devices are thermocouples or thermistors that output a voltage that may be calibrated with a temperature or reading and may be compared to a fixed threshold voltage.
For example, in an automotive power module, such as a power half bridge, temperature is measured to protect the silicon components from heating beyond a rated temperature limitation and for implementing temperature-dependent strategies in the power control system. In conventional systems, the temperature sensor is usually affixed on the outer surface of the power module. However, the accuracy and responsiveness of temperature measurements decrease with the following:                1. distance between the sensing element and the power devices, such as power switching transistors;        2. the quality and conductivity of the bond between the sensor and power devices; and        3. thermal resistance from insulating materials inserted between the sensing element and conductive or semiconductive elements of the power devices to prevent stray voltages from interfering with the signal of the sensing element.        
Thus, it is preferred to place the sensing element as close as possible to the silicon. It is also preferred to have a highly conductive path between the power device and the sensing element, but for the sensing element to be insulated electrically from the high voltages of the power devices, reducing the signal to noise ratio by preventing noise coupling with the control system. The lack of responsive and accurate temperature measurements affects overall device performance. Unresponsive measurements may allow power devices to inadvertently enter an over-temperature condition, which causes damage to the power devices. Alternatively, designers of the power device may adjust the temperature rating of the power device downwards to accommodate a slower response. By reducing the temperature rating, the designers reduce power device efficiency by unnecessarily throttling back the power device at temperatures lower than the actual temperature that is safe for operation of the power device.
Recently, International Rectifier has begun integrating temperature-sensing diodes as part of its power transistor packages, such as in part no. IRLBD59N04E, the data sheet of which is incorporated herein by reference in its entirety for use as background. The date sheet shows two antiparallel, electrically isolated poly-silicon diodes integrated in a MOSFET package.
Some integrated, temperature-sensing diodes have been integrated on the same silicon, semiconductive substrate as a processor, such as the temperature-sensing diode of U.S. Pat. No. 6,363,490. However, a diode on the silicon substrate is not sufficiently isolated for use with power transistors. Instead, the high voltages of power transistors cause excessive noise, which may mask the temperature-dependent signals and may delay responsive measurements.
U.S. Pat. No. 5,049,961 discloses a temperature-sensing diode that is formed directly on a silicon substrate without a dielectric layer, which has a constant current and must have a high enough voltage level to overcome noise interference from the power transistor. Thus, the temperature-sensing diode uses more power than an isolated sensing device that operates at a lower voltage level. U.S. Pat. No. 4,896,196 discloses a temperature-sensing diode that is formed directly on a silicon substrate with a dielectric isolation layer such as SiO2 isolating the diode from the silicon substrate. However, the location of the diode for both of these integrated sensing devices is limited to the surface region of the power device by conventional processing considerations. In addition, the location may interfere with heat removal from the power device by increasing thermal resistance to a heat sink on the surface of the power device package, for example. Also, U.S. Pat. No. 5,726,481 (the '481 Patent) discloses that such integrated sensing devices often experience noise induced false indications, especially as the thickness of the layer of insulation between the semiconductive substrate and the temperature-sensing diode is reduced to improve thermal response. Thus, the '481 Patent recommends shielding the sensing device by electrically connecting a conductive layer overlying substantially all of the device and the semiconductive silicon substrate underlying the sensing device. However, this prevents the attachment of independent leads to the anode and cathode of the temperature-sensing diode to isolate the sensing device from the power device. Instead, the '481 Patent teaches connection of one electrode of the sensing device to the power device ground.